1. Field of Invention
This invention relates to a method for making solar grade silicon wafers.
2. Description of the Related Art
Silicon crystal wafers for solar cells typically are made by heating, melting, and crystallizing essentially pure lumps of silicon. In a typical example, the silicon lumps are placed and packed in a solar grade wafer mold (“mold”) made of rebonded fused silica. Thereafter, the mold, which sometimes is called a “crucible,” is loaded into a vacuum furnace that has heating elements that are usually made of graphite. The vacuum furnace heats the silicon lumps, causing them to melt. Thereafter, the melted silicon is cooled in a manner that encourages the formation of the silicon crystal wafers. This method has its limitations, and some of these are described below.
The length of the heating-cooling cycle often is 45-60 hours, and this can constrain production. One factor in determining the length of the heating-cooling cycle is the time required to heat the silicon feedstock sufficiently to cause it to melt. The silicon lumps used are typically very coarse, as they have an average size of approximately thirty (30) millimeters. The reason such coarse materials are used is to preserve purity in the melted silicon. It has been found that the process of diminution smears undesirable contaminants onto the surface of the silicon. The packing density of such lumps in the mold is approximately thirty-five (35) percent of perfect packing, which is significantly less than ideal. Consequently, heat is not conducted efficiently through such a feedstock, and additional heating time is required. Given that (1) the heating elements are on the outside of the crucible, (2) the silicon lumps have relatively little physical contact with one another, and (3) the silicon lumps “shadow” one another very heavily; most of the heating occurs by radiation that is accomplished in succession, wherein a relatively exposed silicon lump is heated and that lump then radiates heat to one or more relatively unexposed lumps, i.e., one or more silicon lumps that are “shadowed.” In many instances, a partial pressure of argon gas is used to assist in transferring heat to the silicon feedstock.
During the heating step and the cooling step, two impurities, i.e., silicon carbide (SiC) and dissolved-oxygen complexes (including silicon-oxygen complexes), are produced in the silicon feedstock. These impurities cause a reduction in the yield of usable silicon crystal wafers that can be as high as approximately forty (40) percent. Also, these impurities cause defects in the crystal structure that reduce the efficiency and life of the resulting solar cell. A further yield loss is incurred also by the sawing and slicing of the billet into wafers. After the surface impurities of SiO, SiO2, and SiC are removed, then the remaining billet is cut up to allow slicing. In at least some instances, by the time the impurities are removed and the wafers are sliced down to 150-200 microns, the yield on starting silicon can be as low as 35%. At least a few factors encourage the synthesis of these impurities. First, the high temperatures achieved in the furnace promote the oxidation of its graphite heating elements by reduction of the fused silica with which the graphite is in physical contact, thus creating a partial pressure of CO and CO2. Other components of the vacuum furnace may be composed of graphite as well, including the insulation material, and likewise, may too be oxidized. This oxidation-reduction reaction commonly yields two gases: carbon monoxide (CO) and carbon dioxide (CO2). These gases then react with the silicon feedstock in the mold to yield silicon carbide and dissolved-oxygen complexes. Second, although rebonded fused silica is a very refractory substance, it is permeable by carbon oxide gases (e.g., CO and CO2). Thus, carbon oxide gases access the silicon feedstock by permeating the mold. Third, the packing density of the silicon lumps results in spaces that can be permeated and/or occupied by the carbon oxide gases. The surfaces of the silicon lumps that border these spaces serve as additional loci for the oxidation-reduction reaction that yields silicon carbide and dissolved-oxygen complexes. Finally, it must be acknowledged that vacuum furnaces generally do not create perfect vacuums, allowing atmospheric gases and potentially other gases to enter. Atmospheric gases include oxidizing agents that, as described previously, can result in the production of impurities.